strong from weak funext, close #507
unmarked errata for 8b15ad0
new exercise from Marc, close #521
swap encode and decode in 7.3.12, close #517
unmarked errata for aa9b06e
Merge pull request #531 from mikeshulman/encodedecode

swap encode and decode in 7.3.12, close #517
Merge pull request #530 from mikeshulman/marcsex

new exercise from Marc, close #521
Merge pull request #529 from mikeshulman/strongfunext

strong from weak funext, close #507